A suspected U.S. drone fired two missiles at a vehicle in Pakistan's northwest tribal region on Monday, killing at least four suspected militants, intelligence officials said.

The incident was followed by a second drone strike on a house nearby, but no one was killed in that attack, the intelligence officials said.

Both strikes took place about 25 kilometers (16 miles) west of Mirsanshah, the main town in North Waziristan, one of seven semiautonomous tribal districts in northwest Pakistan.

This is the third known drone attack this year in Pakistan, which has had a tense relationship with the United States after a NATO attack left 24 Pakistani soldiers dead in November.

U.S. officials rarely discuss the CIA's drone program in Pakistan, though privately they have said the covert strikes are legal and an effective tactic in the fight against extremists.

The spate of suspected drone activity suggests a moratorium on strikes, called after the November attack, has ended, said Bill Roggio, a senior fellow with the Center for Defense of Democracies and managing editor of The Long War Journal. The center is a think tank that focuses on anti-terrorism policies.

"One strike would have been, 'All right, it's a target of opportunity, we have to take it,'" Roggio said. "Two might be the same thing. My experience with these things is that three means the program is back on."

Roggio predicted drone strikes will continue, but not as frequently as at the height of the drone program, when he said the United States was striking targets almost every third day.